I'm not familiar with Yahoo Site Builder so I'll just try to explain about SimpleViewer.
SimpleViewer and svManager are two different programs that work together. SimpleViewer gives you a very nice gallery and the basic version is free. SvManager is an optional extra that makes it easier to manage your gallery, upload new images, sort them, add captions etc. etc. SvManager costs $35.
SvManager requires PHP support on the server. You can read the detailed requirements in the user manual.
So I'd recommend giving it a try with the free version of SimpleViewer for starters.
Folks who are using SimpleViewer on it's own rather than with svManager have their own forum. I'm not trying to throw you out of this forum but you might get more help over there.
SimpleViewer Support Team.